Friction Spot Joining of High Strength Steel Sheets for Automotives

In this study, friction spot joining (FSJ) was applied to dual-phase steel DP980 using a newly-developed silicon nitride (Si3N4) tool with a threaded pin, after which microstructure and strength of the joint were examined. The cross-sectional microstructure was distinctly classified into four zones which experienced the different heat cycles and deformation histories during FSJ. Electron probe micro analyzer revealed that silicon, oxygen and nitrogen were involved in the alternate band patterns observed in the stir zone, which could be attributed to contamination from the tool. The contamination was prevented by coating of the tool surface and the use of Ar shielding. The mechanical tests showed that the contamination reduced the joint strength, especially the cross tensile strength.